"The war will be shifting to Kenya, if they kill a Somali girl we kill a Kenyan girl," Fuad Mohamed Khalaf, one of the Shebab's most senior commanders, said in a radio broadcast.
"We are urging all the Muslims in Kenya... To fight the government of Kenya inside that country, because Kenyans killed your people including children," Khalaf said in a speech broadcast on the Shebab's Radio Andalus.
Kenyan troops crossed into southern Somalia in 2011 to fight the Shebab, later joining the now 22,000-strong UN-mandated African Union force battling the Islamists.
"When their soldiers and war planes kill your people, God permits you to retaliate accordingly, we will fight the Kenyans," Khalf said, viewed as second in importance only to Shebab chief Ahmed Abdi Godane.
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The Shebab, who claimed responsibility for the September 2013 attack on Nairobi's Westgate mall in which at least 67 people were killed, have also been blamed for a string of grenade blasts and killings.
Last week a double bomb attack in a Nairobi market left 10 people dead and scores wounded, while the United States has said it was preparing to cut staff levels in Kenya because of the mounting threat of attacks.
Khalaf said the Shehab had trained fighters and vowed more would be sent to carry out attacks inside Kenya.
"We have trained the people... They are the ones who carried out the Mandera attack," he said. "More are going to be sent soon."
The United States have offered a $5 million bounty for Khalaf, who holds both Somali and Swedish nationality.
Khalaf, who the US says is both a Shebab military commander and key fundraiser, reportedly spent over a decade in the Swedish capital Stockholm.